A magnetic resonance imaging apparatus (hereinafter referred to as MRI apparatus) is for obtaining magnetic resonance images (hereinafter referred to as MR images) that represent the physical properties of an object to be examined by applying magnetic nuclear resonance phenomenon being generated in atomic nuclei of atomic elements which structure the body of the object upon irradiating electromagnetic waves to the object being placed in homogeneous static magnetic field and detecting nuclear magnetic resonance signals (hereinafter referred to as NMR signals) from the object.
Consequently, an MRI apparatus is provided with a static magnetic field generating source for generating static magnetic field in imaging space in an imaging space for arraying the spin direction of hydrogen nucleus (proton) of the object in a predetermined direction. As for the static magnetic field source to be utilized for MRI apparatuses, the ones using superconducting magnets other than permanent magnets are widely commercialized. A general example of a superconducting magnet would be one that generates a static magnetic field by comprising a liquid helium container filled with liquid helium in the vacuum container vacuumized inside thereof, further arranging a superconducting coil therein, and electrifying the superconducting coil which is rendered superconductive state. Such a superconducting magnet is referred to as a liquid helium-cooled superconducting magnet.
There is a technique in relation to a liquid helium-cooled superconducting magnet for re-condensing the vaporized helium gas using a cryo-cooler (freezing machine) for the purpose of compensating the evaporation of liquid helium due to minute inflow of heat into the liquid helium container (for example, Patent Document 1).
Patent Document 1: JP-A-2001-238864.
After examining the above-mentioned conventional technique, the inventors of the present invention have found some problems described below.
In the MRI apparatus according to Patent Document 1 using a cryo-cooler to re-condense the evaporated helium gas, there are cases in which the cryo-cooler is detached for the purpose of maintenance. In such cases, problems are caused when air flows into an insertion port of the cryo-cooler, by constituents in the air being solidified in the vicinity of the insertion port of the helium container and blocking the insertion port. In concrete terms, for example, by constituents of the air being solidified in the closest side to the helium container of a pipe for inserting the cryo-cooler being connected to the liquid helium container, there are times that the cryo-cooler is blocked off from the inside of the helium container when it is reattached. In this case, the vaporized helium gas cannot reach the cryo-cooler, and a problem is caused that the gas cannot be easily re-condensed.
There are some conventional methods such as spraying helium gas of normal temperature onto the insertion port of the cryo-cooler or inserting a heater and dissolving the solidified air. However, using these methods by spraying helium gas of normal temperature to the insertion port of the cryo-cooler or putting in a heater creates additional problem of an increase in temperature of the liquid helium container which increases the possibility for quenching the superconducting coil therein.